Physics Factsheet

www.curriculum-press.co.uk Number 151

Telescopes
When we think of telescopes, we automatically think of optical telescopes (sensitive to the visible light region of the electromagnetic spectrum). And if we have a special interest in astronomy, we may also consider radio telescopes (sensitive to the radio region of the spectrum). However sources in the universe emit signals across almost all of the electromagnetic spectrum, and information can be gained from studying the whole spectrum. The reason for limiting most of our studies to these two regions has to do with our atmosphere.

Gamma ray telescopes
The universe has sources that can generate inconceivable amounts of energy. Black holes, merging neutron stars, high-speed streams of hot gas, pulsars, and even solar flares, are examples of sources of gamma radiation. Gamma ray telescopes can also be used to study the cosmic rays approaching the Earth. Cosmic rays are not really rays at all – they are extremely high-energy particles, originally thought to be e.m. radiation beyond the gamma ray spectrum. When they collide with particles in the Earth’s atmosphere, gamma rays are emitted. A gamma ray telescope on the Earth’s surface can detect these gamma rays using a set of large reflecting mirrors. However most gamma ray telescopes must be mounted above the Earth’s atmosphere, as gamma rays are absorbed by the atmosphere. The Fermi gamma ray telescope was launched by NASA in 2008.

Atmospheric Windows
Much of the electromagnetic radiation approaching the Earth is absorbed by the atmosphere.

Atmospheric absorption

Example1. Why is the focusing of gamma rays particularly difficult? Answer: Gamma rays are very penetrating, and tend to travel through most materials, rather than reflecting off the surface. In the HESS telescope, the reflecting mirror is made of aluminised glass with a quartz coating. This mirror is made of 382 round mirror segments (for cost reasons). Example2. A gamma ray may have an energy of 1×1010 eV. Find × its wavelength, and compare the energy to that of a photon in the visible light region. Answer (a) E = 1×1010 × 1.6×10-19 = 1.6×10-9J as E = hf and f = c/λ then λ = hc/E = 1.2×10-16m (b) for visible light, λ = 5×10-7m E = hc/λ = 4.0×10-19J

Gamma rays

X-rays UV

IR

Radiowaves

visible light

Only visible light and radio wavelengths can penetrate the atmosphere to any extent. So historically these are the two regions of the e.m. spectrum that we tend to study the most. However, with our ability to put objects into orbit (more-or-less above the atmosphere), and to control these objects and communicate with them, it is now possible to extend our study across the spectrum. The placement of a telescope is very dependent on the effect of the atmosphere on the range of wavelengths for which the telescope is designed to work. In this factsheet we are going to survey briefly the different types of telescope we can use today and for each telescope study some of the following points: • • • • • • the reason for looking at each region of the spectrum the type of telescope used where we position the telescope how it receives and focuses the radiation the strength and resolution of the final image what we learn from it

X-ray telescopes
X-rays are also absorbed by the Earth’s atmosphere. X-ray telescopes must be positioned in space. The XMM-Newton telescope was launched by the European Space Agency in 1999. It was intentionally put into a very elliptical orbit. This means that most of its orbit is well outside the Earth’s atmosphere, leading to less distorted images.
Limit of Atmosphere Earth Orbit

We will start from the high-energy end of the e.m. spectrum.

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151. Telescopes

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FUSE was launched by NASA in 1999, and was finally decommissioned in 2007, when its pointing mechanism failed and couldn’t be restarted. Ultraviolet light is difficult to work with because there are no mirror coatings that work over the whole range. Instead FUSE had four mirror segments, two with a coating that worked for short UV wavelengths, and two that worked for long UV wavelengths. The UV light focused by FUSE was then separated into its spectrum by a reflection grating. Because of the large number of atomic absorption and emission lines in the UV spectrum, FUSE enabled many studies of chemistry and chemical evolution from sources within and beyond our galaxy.

Example 3 What second effect of the elliptical orbit is useful? Answer: Satellites in elliptical orbits will decelerate as they move away from the Earth, and then accelerate as they approach the Earth (due to the Earth’s gravitational field). This means they will travel slowest when they are furthest from the Earth, extending the time available for the most accurate image work. One of the aims of XMM-Newton was to study the X-ray emission from black holes. And astronomers have used XMM-Newton to discover a new black hole in the intermediate mass range. These intermediate mass black holes had been predicted, but none had been observed before. Stellar-mass black holes (three to twenty times as massive as the Sun) have been observed, and supermassive black holes (millions of times as massive as the Sun) have been observed. But this was the first detection of an intermediate mass black hole. There are three X-ray telescopes on XMM-Newton. Mirrors are used for focusing the image, but there is a problem with X-rays being too penetrative. The mirrors are set up so that the X-rays approach at a grazing angle of about 30 degrees, to encourage maximum reflection and minimum penetration by the rays. This is very different from conventional optical reflecting telescopes.
mirror segment x-ray image x-ray mirror segment

Optical telescopes
Optical telescopes are the ones we are all most familiar with, and you will have learned a considerable amount about them already. Consequently we will mention them least in this factsheet. Optical telescopes can be positioned on the Earth’s surface, but the atmosphere does cause problems. A certain amount of absorption of visible light takes place in the atmosphere, varying temperatures in the atmosphere cause refraction leading to distortion, atmospheric pollution causes distortion, light pollution reduces the sharpness of signals, and it has even been known to be occasionally cloudy over the UK. Observations are also, of course, restricted to night-time. A telescope in space, like the Hubble telescope, can produce far brighter images, with much less distortion and much better resolution. Optical telescopes can use either mirrors or lenses as their primary focusing instrument. However lenses cause chromatic aberration (the splitting of the colours), and this is a problem that cannot easily be overcome. Violet light has a larger index of refraction than that of red light, and is focused more quickly.
white light white light

Ultraviolet telescopes
Once again it is necessary to launch ultraviolet telescopes into space, to avoid the absorption of the UV rays by the Earth’s atmosphere. A particular point of interest for UV observation was the study of deuterium in the Universe. A deuterium atom is a hydrogen atom with a neutron as well as a proton in the nucleus.
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violet p + p + n mirror Hydrogen Deuterium red

Deuterium was formed in the instant after the Big Bang, but is also formed in the interior of stars. However, in stars, the high temperature conditions persist for a very long time, allowing lighter atoms, like deuterium, to be converted into heavier elements. Scientists think that as the matter in the Universe is recycled through the generation and life cycles of stars, the proportion of deuterium in the Universe should be decreasing. The study of this was one of the aims of the Far Ultraviolet Spectroscopic Explorer (or FUSE) telescope.
Big Bang Amount of deuterium Today

Example 4. telescopes.

Suggest other problems with large lenses in

Answer: Large lenses are also much heavier and more difficult to support than mirrors without sagging occurring. (They must be supported at their edges rather than from behind.) Sagging will change the shape and curvature of the lens so its focal length will change too. Exam Hint:- You should be aware of advantages and disadvantages of mirrors and lenses in telescopes. All of the larger telescopes use reflection.

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151. Telescopes
Infrared telescopes
The Spitzer infrared telescope was launched by NASA in 2003. (Infrared rays are absorbed by the atmosphere.) This reflecting telescope has an 85cm diameter mirror, with the recording instruments cooled to 5.5 Kelvin. The interesting point about its orbit is that it does not orbit the Earth. It has been put into orbit about the Sun – we say it is in a trailing orbit behind the Earth.
Telescope assembly Earth

Physics Factsheet

Another advantage of positioning the telescope at the South Pole is that the Pole is 2.8km above sea level, so the atmosphere is relatively thin anyway. The South Pole Telescope (SPT) is a 10-metre reflector, designed to measure microwaves of wavelength less than 1mm. The large diameter of the telescope mirror increases the signal received, and improves resolution (see the discussion in the Radio Telescope section). The diameter of a telescope affects both the magnitude of the signal received and the resolution of the image.

sun

The SPT is used to locate distant clusters of galaxies. It uses a sophisticated technique involving distortions in the Cosmic Microwave Background that pervades the Universe.

Example 5. Why might this “trailing orbit” be superior to an orbit about the Earth for an infrared telescope? Answer: The key here is the fact that infrared radiation is heat radiation. To avoid thermal interference the telescope instruments must be kept very cold. However the Earth radiates infrared. By keeping the telescope in orbit about the Sun, and using a solar shield to protect it from the Sun’s radiation, the amount of cooling required is very significantly reduced, leading to a greater lifetime for the assembly. The Spitzer telescope detects infrared radiation from cooler stars that don’t emit much visible light, and has also detected the presence of planets in other solar systems from the infrared they are emitting. In addition, infrared radiation can penetrate dense gas clouds in space that block visible light. The Spitzer telescope enables us to “see through” these clouds. Spitzer ran out of its liquid helium coolant on May 15, 2009, and it is now warming up to the ambient temperature (30K). More research is planned for the telescope, operating in its “warm” phase.

Radio Telescopes
Much of the radio wave spectrum coincides with an atmospheric window – radio waves are not absorbed to any significant extent by the atmosphere. So radio telescopes can be sited on the Earth’s surface. This allows very large structures to be built. The Lovell radio telescope near Manchester has a reflector diameter of 76m. This large diameter is very important for improving the resolution of sources being observed. For a circular aperture, two point sources can be observed as separate, if the angle between them is greater than θ, where θ is the angular resolution, defined by: sin θ = 1.22λ / D where λ is the wavelength of the incoming radiation, and D is the diameter of the aperture. Example 7. Find the angular resolution of the Lovell telescope when observing radio waves of wavelength 21cm (the hydrogen line). Answer: sin θ = 1.22×21×10-2 / 76 = 3.4×10-3, θ = 0.19 degrees This is very poor resolution compared to optical telescopes studying visible light:

Microwave telescopes
Microwave radiation is absorbed by water - that is how microwave ovens work. So the water vapour in the atmosphere will absorb incoming microwave radiation. Example 6. Why is a major microwave telescope sited at the South Pole? Answer: Very cold air cannot hold much water vapour. This limits the ability of the atmosphere at the South Pole to affect the incoming microwaves. See the graph below. The graph shows how cold air is always very dry. The ability of air to hold water vapour increases rapidly as the temperature rises:

Optical image telescope

Radio image telescope

One way of improving resolution is to electronically link a number of small dishes, effectively increasing the diameter.

60 maximum 40 H 2O (gm-3) 20 D

However this does not significantly improve the total signal strength received, as this depends on the total area of the collector. Radio telescopes can be used to monitor radio communication from space probes, etc, and can also be used to study cooler radio sources in space, which do not emit shorter wavelength radiation e.g. visible light.

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151. Telescopes
Questions
1. Suggest three advantages of using optical telescopes. 2. (a) Find the resolving angle for a 3.0m optical telescope observing light of wavelength 5.5×10-7m. (b) Compare this to the resolving angle of the Lovell telescope. (c) How would the final image produced by this telescope compare with that produced by the Lovell telescope? Explain your answer. 3. A certain X-ray telescope is designed to be sensitive to a wavelength of 1.2×10-10m. Find the energy (in eV) of photons associated with this wavelength.

4. Compare the signal strength received by an 18m radio telescope compared to a combined series of 8 dishes, each with a diameter of 50cm. 5. The Very Large Telescope (VLT) is positioned in the Atacama desert in Chile. It is a combination of four Cassegrain telescopes, each of which has a diameter of 8.2m. The VLT detects electromagnetic radiation of wavelength 200nm - 20 micrometres. (a) Show that the VLT has a similar light-collecting power to that of a single telescope with a diameter of 16m. (b) The VLT achieves an angular resolution similar to that of a telescope of 100m diameter. Calculate the maximum angular resolution of the VLT.

Answers
1. Many strong optical sources in the Universe, good resolution of signal sources, can be situated on the Earth’s surface meaning relatively cheap to build, easy to control and flexible with regard to attaching various instruments, etc. 2. (a) θ = 1.3×10-5 degrees (b) This is a much smaller angle than the resolving angle of 0.19 deg for the radio telescope. (c) There would be better resolution than with the Lovell telescope. The exact positions of each source can be more accurately identified, and sources with small angular separation can be identified separately rather than being blurred together. 3. Energy = 1.0×104eV or 10keV 4. Area big dish = π×92 = 250m2. Total area small dishes = 8×π×.252 = 1.6m2 Signal strength with big dish is 150 times greater. 5. (a) (light collecting power is proportional to area) Area of four telescopes of diameter 8.2m = 4π (d/2)2 = 211m2 Single telescope of this area has a diameter = 2√(A/π) = 16.4m (alternatively) Light collecting power is proportional to area, area is proportional to diameter2 Therefore diameter2 of four telescopes = diameter2 of single telescope 4×(8.2)2 = d2 d = 16.4m (b) use of θ = 2 200 × 10-9 gives θ = = 2.0 ×10-9 rad d 100

Acknowledgements: This Physics Factsheet was researched and written by Paul Freeman The Curriculum Press,Bank House, 105 King Street,Wellington, Shropshire, TF1 1NU Physics Factsheets may be copied free of charge by teaching staff or students, provided that their school is a registered subscriber. No part of these Factsheets may be reproduced, stored in a retrieval system, or transmitted, in any other form or by any other means, without the prior permission of the publisher. ISSN 1351-5136